# LidarWhisk1

Keywords: lidar

## Summary

This demonstrates a nadir viewing LIDAR system that whisk scans across the scene. In this demo, we shoot 12 pulses as the system scans.

## Details

This demo models a LIDAR system with a simple transmitter and a 128 x 128 receiver array with 2 us long listening window and 1 ns time resolution (resulting in 2001 time bins). The multi pulse collection is then run through both a Linear mode and Geiger mode detector model to produce ASCII/Text XYZ point clouds. The LIDAR detector model has a graphical interface available under Tools → Lidar Tools in the Simulation Editor, as well as a command-line interface.

## Important Files

This section highlights key files important to the simulation.

### Geometry and Materials

The scene is comprised of a single maple tree (see `geometry/Red_Maple_1.gdb`) planted at the Scene ENU origin a flat plate. The reflectance properties are spectrally flat and not realistic (see `materials/demo.mat` and the optical property files in the `materials` folder).

### Platform

This section summarizes the properties of the transmitter and receiver components of the laser radar system used in this demo.

#### Transmitter

    Laser wavelength: 1064 microns
    Pulse repetition rate (PRF): 2000 Hz
    Pulse length: 2 ns
    Pulse energy: 1 mJ

#### Receiver

    Focal length: 400 mm
    Array size: 128 x 128 elements @ 50 micron pitch
    Range gate width: 2 us
    Range gate resolution: 1 ns

## Setup

### Run the Radiometry Simulation

To run the simulation, perform the following steps:

1. Run the DIRSIG `demo.sim` file

This produces the file `demo.bin`.

### Run the Analysis Tool

It is always useful to verify the link budget (the number of photons arriving onto the pixels during a pulse). The BIN Analysis tool can be used to examine and summarize the radiometric values and supporting metadata in a BIN file. This tool can be run via the Analyze button in the detector model interface or via the command-line:

    $ bin_analyze demo.bin
    ...
    [header summaries deleted for documentation purposes]
    ...
    Pulse #1:
        Minimum total photons in a pixel = 9.09949
        Maximum total photons in a pixel = 24.3864
        Pixels with zero total photons = 0/16384 (0%)
        Average background photons per pixel = 3.05467e-05
        Average total photons per pixel = 17.8523

    ...

    Pulse #12:
        Minimum total photons in a pixel = 9.09505
        Maximum total photons in a pixel = 24.3765
        Pixels with zero total photons = 0/16384 (0%)
        Average background photons per pixel = 2.96592e-05
        Average total photons per pixel = 17.845

    Complete!

### Run the Linear-mode (Lm) Detector Model

To generate a linear mode detector model on the spatial x spatial x time data captured in the `demo.bin` file, the detector model should be run in linear mode with the following parameters:

- A delay time of 2 ns

- A dead time of 2 ns

Generating the Linear-mode point cloud product can be accomplished using the graphical interface to the LIDAR Detector Model or via the command-line:

    $ simple_processor --input_filenames=demo.bin \
        --model=linear \
        --delay_time=2.0e-09 --dead_time=2.0e-09 --return_count=3 \
        --output_format=ascii --output_filename=demo_linear.xyz

This produces the "demo_linear.xyz" file, which is an ASCII/text point cloud, that can be visualized in most 3D point cloud viewers or generic plotting tools.

### Run the Geiger-mode (Gm) Detector Model

The Geiger-mode detector model simulation uses detector specifications commonly found in commercial Geiger-mode array devices:

- Photon detection efficiency (PDE) of 0.5

- Dark count rate (DCR) of 5 kHz

Generating the Geiger-mode point cloud product can be accomplished using the graphical interface to the general detector model or via the command-line:

    $ simple_processor --input_filenames=demo.bin \
        --model=geiger --pde=0.5 --dcr=5.0e+03 \
        --output_format=las --output_filename=demo_geiger.las

Note | The large average link budget (nearly 30 photons/pixel) is significantly higher than an actual Geiger-mode system would use. Expected link budgets for a Geiger-mode system is usually less than 1 photon/pixel.

## Results

Visualizations of Linear-mode and Geiger-mode point clouds in GNUplot (a simple 2D and 3D plotting tool) are shown below:

Figure 1. The linear-mode 3D point cloud visualized in GNUplot
Figure 2. The geiger-mode 3D point cloud visualized in GNUplot